The salmonella strain that led to the recalling of one million bars of Cadbury's chocolate may have contaminated many more of the company's brands, the Food Standards Agency has told the Guardian.

Fears have been raised because the mix used in the seven products that were taken off the shelves was also the base ingredient in other brands.

Thirty others are now being tested and the FSA has not ruled out the possible withdrawal of others. "There may be contamination in other Cadbury products," said spokesman Justin Everard.

"We have discussed it with them. They are testing all their finished products and the local authority is testing as well. If more products come up positive we will expect them to recall them too."

Cadbury first detected a rare strain of salmonella in samples of its chocolate crumb - a sugar, milk and cocoa mix - in January. The company told the FSA of the contamination on June 19. The crumb, made at its Herefordshire factory, is used as the base for products made at the Cadbury factories near Birmingham and Bath.

The FSA said it understood that the crumb from which the salmonella-positive samples came went into a large number of Cadbury products. Tests on the final products found samples from seven brands positive for Salmonella Montevideo, and so those seven brands were recalled. Although the FSA believes that the crumb was contaminated over a period, there has been no recall of other products.

Birmingham council's food safety team confirmed it was currently testing about 30 Cadbury brands other than those recalled already that had been made from crumb stored in the silo into which the contaminated product had been put.

"The crumb goes into a very large number of Cadbury products. There is the inevitable question, is there a danger it is in other things. They've tested all their final products and we believe the recall is proportionate," Mr Everard said. Although Cadbury says it is confident that the source of the contamination has been removed, the FSA said it was not certain where the salmonella came from.

Cadbury's director of communications Andrea Dawson-Shepherd said the contaminated crumb was "only detected in the products recalled". "We are testing product lines four times a day, and environmental health are checking so they can feel as confident as we do about our testing regime. We have tested all products and we've found no salmonella." she said. Cadbury said it had traced the salmonella source to a leaking waste pipe at the Herefordshire factory which had been fixed.

The contamination was uncovered when the health protection agency noticed an increase in reported cases of salmonella Montevideo. A private laboratory used by Cadbury had separately sent nine anonymous samples of the bacteria to the HPA for identification and the HPA noticed a possible link. Since the laboratory declined to identify its client, the HPA informed the FSA on June 16 of its concerns. The FSA then contacted the lab, and on July 19 Cadbury admitted the salmonella Montevideo samples were from its chocolate products. The company said the salmonella had been detected at such low levels it had decided it was not a risk and had therefore not notified the FSA. The FSA said it was surprised by the delay.

Kath Dalmeny, policy expert at the campaign group The Food Commission said: "It seems Cadbury has been arrogant enough to rely on its reputation to get it through a crisis rather than taking immediate action."